The purpose of this research is to study the cellular changes accompanying the differentiation of the human fetal ovary and testis by electron microscopic examination of gonads obtained following therapeutic abortion. Beginning with testicular differentiation (6 weeks), the stages to be studied will continue until the initiation of follicle formation in the ovary (20 weeks). The main problems involved are: (1) germ cell differentiation; (2) differentiation of "supporting" (granulosa, Sertoli) cells; (3) analysis of the morphologic relationship between germ cells and supporting cells; (4) formation of seminiferous tubules and ovarian follicles; (5) development of steroid-secreting (interstitial, theca) cells; (6) evidence of abnormal cellular differentiation. An anticipated goal of the research is the preparation and publication of an atlas of the ultrastructure of human gonadal development. The study will also be correlated with radioimmunoassay determinations of luteinizing hormone, follicle-stimulating hormone, testosterone, progesterone, estrone, and estradiol-17 beta.